Is clean air becoming a luxury? According to the latest annual "State of the Air" report (pdf) from the American Lung Association, 22 of the 25 most polluted cities in the U.S. experienced higher levels of air pollution in the past year. All told, the report finds that nearly 148 million people, or about half of the U.S. population, live in areas where they are exposed to unhealthy levels of air pollution on a near-daily basis.

The report, which covers data from 2010 through 2012, found that ozone-laden smog has gotten worse in most major metropolitan areas. The report links the increase in ozone to climate change. "Weather played a factor," the association wrote in the report's introduction. "The warmer summers in 2010 and 2012 contributed to higher ozone readings and more frequent ozone days. Sunlight and heat create conditions that increase the risk of high ozone levels."

Los Angeles was once again found to be the worst city for ozone pollution. Like many other cities on the list from the American Lung Association (ALA), Los Angeles experienced more days with unhealthy levels of ozone pollution than healthy days. Other cities with more unhealthy days on average included Bakersfield, California; Las Vegas; Houston; New York City; Tulsa, Oklahoma; Cincinnati; Philadelphia; and Cleveland, Ohio.

On the other hand, the ALA found that year-round particulate air pollution improved somewhat over this period. "Among the 25 cities with the worst year-round levels of particle pollution, 18 had lower levels in 2010-2012, while five recorded higher annual levels and two cities maintained the same level," the report reads. "However, all of the most polluted cities continue to have year-round particle levels that violate health-based standards."

Cities and metropolitan areas with the worst levels of particulate pollution included Fresno-Madera, California; El Paso-Las Cruces (Texas and New Mexico); Phoenix; Birmingham, Alabama; and San Diego. Numerous cities improved over the previous report — including many cities on the ozone list, including L.A., Bakersfield, Philadelphia and Cleveland.

The ALA report finds that "more than 147.6 million Americans live in the 330 counties where they are exposed to unhealthful levels of air pollution in the form of either ozone or short-term or year-round levels of particles." Elevated ozone levels have been linked to health problems such as asthma, cardiovascular disease and lower birth rates. Ricks from particle exposure include heart attack, stroke and asthma. Long-term exposure is linked to increase risks of premature death. Previous studies have linked 1 in 8 deaths worldwide to air pollution.

There were a few bright spots in the report, though, as the ALA named the country's four cleanest cities: Bangor, Maine; Bismarck, North Dakota; Salinas, California; and the Cape Coral-Fort Myers region of Florida. Other high-ranking cities for clean air included Anchorage, Alaska; Burlington, Vermont; and Flagstaff, Arizona.